Saturable binding of sulfobromophthalein (BSP) to hepatic canalicular and plasma membranes has been demonstrated. Furthermore, proteins with high affinities for bilirubin and BSP have been isolated from hepatic cell surface membrane fractions suggesting that carrier-mediated transport mechanisms may be of importance in the hepatic uptake and excretion of these organic anions. In studies of bile salt transport it has been shown that microfilaments play a role in hepatic active transport systems. Bilirubin kinetics have been studied before and after splenectomy in 14 patients with hereditary spherocytosis. In 7 hepatic clearance of bilirubin was normal before splenectomy and was unaltered after splenectomy. In the remaining 7 hepatic clearance of bilirubin was subnormal before splenectomy and was increased after splenectomy; in this group hepatic UDP glucuronyl transferase activity was reduced. These findings suggest the existence of a latent state of Gilbert's syndrome which can be unmasked by hemolytic stress.